Adaptive cancellation technology is widely used in the fields of audio engineering and in many other areas of signal processing as well. As applied to audio engineering, adaptive cancelling is often used for echo cancellation and noise suppression. The general principle is to remove an unwanted component of a main signal, such as an audio signal, by obtaining a reference version of the unwanted signal, and feeding this unwanted signal back into an electrical circuit that uses an adaptive cancellation processor to detect the unwanted reference signal in the main signal. The adaptive cancellation processor uses the reference version of the unwanted signal to find the corresponding unwanted signal in the main signal, makes suitable timing, phase, amplitude and other corrections to obtain a precise match, and then subtracts the reference unwanted signal from the main signal, thus producing a cleaned up version of the main signal that, for example, has the unwanted echo or noise background suppressed. The methods often rely on an adaptive filtering algorithm or device that minimizes the mean square errors between the main signal, and the reference unwanted signal.
Examples of adaptive cancelling technology can be found in various publications. These include Kuo et. al., “Active Noise Control: A Tutorial Review”, Proceedings of the IEEE 87(6), June 1999, pages 943-973; Naylor et. al., “Adaptive algorithms for sparse echo cancellation”, Signal Processing 86 (2006), pages 1182-1192; Gilloire and Vetterli, “Adaptive Filtering in Subbands with Critical Sampling: Analysis, Experiments, and Application to Acoustic Echo Cancellation”, IEEE Transactions on Signal Processing 40(8), August 1992, pages 1862-1975; and Wang, “Low-Power Filtering via Adaptive Error-Cancellation”, IEEE Transactions on Signal Processing 51(2), February 2003, pages 575-583.
Turning to a different area of technology, Cable television (CATV), originally introduced in the late 1940's as a way to transmit television signals by coaxial cables to houses in areas of poor reception, has over the years been modified and extended to enable the cable medium to transport a growing number of different types of digital data, including both digital television and broadband Internet data.
Over the years, this 1940's and 1950's era system has been extended to provide more and more functionality. In recent years, the CATV system has been extended by the use of optical fibers to handle much of the load of transmitting data from the many different CATV cables handling local neighborhoods, and the cable head or operator of the system. Here the data will often be transmitted for long distances using optical fiber, and the optical (usually infrared light) signals then transformed to the radiofrequency (RF) signals used to communicate over CATV cable (usually in the 5 MHz to about 865 MHz frequencies) by many local optical fiber nodes. Such systems are often referred to as hybrid fiber cable systems, or HFC systems. The complex electronics that are used by the cable operator to inject signals (e.g. data) into the system, as well as extract signals (e.g. data) from the system are often referred to as Cable Modem Termination Systems or CMTS systems.
In a typical HFC system, at the various optical fiber nodes, the optical fiber signals are transformed back into RF signals and are then carried by the various neighborhood CATV coax cables to various households. Unlike fiber, which can carry optical signals for extensive distances without significant signal strength attenuation, the RF signals attenuate fairly rapidly as a function of distance over the CATV coax cables. This attenuation versus distance function increases as the frequency of the RF signals increases. For example, using RG-59 cable, at 10 MHz, the RF signal attenuation versus distance is about 1.1 dB/100 feet, at 100 MHz, the RF signal attenuation versus distance is about 3.4 dB/100 feet, at 400 MHz, the attenuation rate is 7.0 dB/100 feet, and at 1000 MHz (1 GHz), the attenuation rate is 12 dB/100 feet. Other types of coax cables, such as RG-6 cables, have lower attenuation versus distance characteristics, but the same sort of attenuation problem still exists.
Thus, in order to maintain the RF signal of the various upstream and downstream signals while traveling over neighborhood CATV coax cables, neighborhood CATV systems typically employ various active (powered) devices, such as powered forward and reverse (bidirectional) RF amplifiers and the like. At present, using CATV systems that often have a maximum frequency of about 550 or 850 MHz, these active devices are often spaced about every 1000 feet.
Each active device can have several (e.g. 1-4) neighborhood CATV sub-cables connected to it, and often to maintain RF power over cable distances of several thousand feet, more than one (usually 1-3) active devices can be connected along a single stretch of coax cable. As a result, at a neighborhood level, the coax cable wiring pattern of CATV systems often has a “tree” like structure, where the branches of the CATV coaxial cable tree spring off of the various active devices. The first or main CATV coax cable that connects to the RF signal originating from the optical fiber node is often referred to as the “trunk” cable, and the various coax cables that split off of the trunk cable are often referred to as branch cables, and the branch cables in turn can have other branch cables splitting off of them as well. As the various trunk and branch cables cover the local neighborhood, and generally situated in between the various active devices, various taps, splitters, and drops on the neighborhood or “trunk” CATV cable connect various households to the CATV cable. In order to provide power for the various active devices, often the CATV coax cable system will carry electrical power as well. As might be expected, the process of negotiating easements and right of way to route the neighborhood CATV cables is burdensome, however this process has been going on for over 50 years in various parts of the country, and by now is well established.
At present, in United States CATV systems, the 5-42 MHz frequency region is reserved for upstream communications back from the various cable modems to the cable plant/cable head end, and the majority of the bandwidth, typically in the 54-547+MHz range (often the upper end extends to 865 MHz and beyond) is reserved for downstream communications from the cable plant to the various households. European CATV systems follow a slightly different scheme where the upstream communications frequencies extend up to the 65 MHz region, and the downstream communications frequencies are typically in the 88 to about 865 MHz range. The intermediate frequencies between 42-54 MHz (US) and 65-88 MHz (Europe) are generally unused due to the filtering switch over in this region. Due to rapid signal attenuation, the higher frequencies above about 750 to 865 MHz (here referred to generically as 1 GHz+ frequencies or wideband frequencies) are seldom used at present.
A more detailed discussion of prior art in this field can be found in parent application Ser. Nos. 13/346,709, 12/907,970, and 12/692,582, the contents of which are incorporated herein by reference.
Prior art work with various types of CMTS systems and fiber nodes includes Liva et. al., U.S. Pat. No. 7,149,223; Sucharczuk et. al. US patent application 2007/0189770; and Amit, U.S. Pat. No. 7,197,045.
As demand for ever more data carrying capacity both downstream (from the cable head or plant to the various clients) and upstream (from various clients to the cable head and plant) has increased, the finite bandwidth (e.g. data carrying capacity) of CATV systems has become ever more constraining. Thus methods to increase the limited upstream and downstream data carrying capacity of CATV cable are of great commercial interest.